Fix tests on Ubuntu
build: make `make clean` remove test/runner.o
win: fix potential HANDLE corruption due to incorrect cast

Closes #738.
build: rename OS to PLATFORM

Rename the OS make variable to PLATFORM, it conflicts with the OS env
var. That is, running `make` when the OS env var is set, may cause
spurious build breakage.

Fixes #737.
Merge pull request #12 from staticfloat/combinedpullrequest

closes #12, closes #11, closes #10, closes #7